The intercostal muscles drive expansion and contraction of the rib cage during respiration. The action of the intercostal muscles has been studied primarily by studying their effect on rib displacement, but their effect on lung volume expansion is still poorly understood. In our previous work, we combined modeling, theory, and experiment to establish a method for evaluating the mechanical advantage of the intercostal muscles. Mechanical advantage was defined as the change in airway pressure per unit force exerted by an active muscle, and mechanical advantage was shown to equal fractional muscle shortening per unit volume expansion of the passive chest wall. We calculated the mechanical advantages of the intercostal muscles of the dog and tested our prediction of the maximum respiratory effect of entire inspiratory muscle groups. The calculations revealed new information about the respiratory action of the inspiratory intercostal muscles. The calculated mechanical advantage of the internal intercostals in the parasternal region (PA) showed a gradient within each interspace with mechanical advantage decreasing with distance from the sternum and a gradient between interspaces with mechanical advantage greater in the third than in the sixth interspace. Gradients in mechanical advantage of the external intercostals were also predicted with mechanical advantage greatest in the dorsal region and decreasing gradually toward the ventral region. We hypothesized that the distribution of the muscle activation during spontaneous breathing might follow the distribution of mechanical advantage. In the proposed research, we will test the calculated values of mechanical advantage by measuring the shortening of the PA and EI as a function of position within interspaces and across interspaces. We will measure the distribution of activation, as a fraction of maximal activation, of PA and EI to test the hypothesis that muscles with greatest mechanical advantage are more active during breathing.